1. Field of the Invention
The present invention relates in general to molecular sensors and methods of identification, and more particularly to a nanosensor detection system for molecular identification based on surface-enhanced Raman scattering (SERS).
2. Description of Related Art
Raman scattering is the inelastic scattering of optical photons by interaction with vibrational modes of molecules. Typically, Raman scattered photons have energies that are slightly lower (i.e., Stokes-shifted photons) than the incident photons with the energy differences related to molecular vibrational energy levels. The energy spectrum of scattered photons commonly comprise narrow peaks and provides a unique spectral signature of the scattering molecule, allowing a molecule to be identified without the need for optical labels or prior knowledge of the chemicals present in the sample. Additionally, the vibrational spectra acquired in Raman spectroscopy are complementary to the vibrational spectra acquired by infrared (IR) absorption spectroscopy providing an additional database for peak assignment and molecular identification.
A drawback of Raman spectroscopy, however, is that the typical molecular cross-sections for Raman scattering are extremely low, on the order of 10−29 cm−2. These low cross-sections often require high laser fluences and long signal integration times to produce spectra with sufficient signal-to-noise. While Raman spectroscopy has been used as an analytical tool for certain applications due to its excellent specificity for chemical group identification, its low sensitivity historically has limited its applications to highly concentrated samples. Background for such a method is described by Lewis, I. R. and H. G. M. Edwards in Handbook of Raman Spectroscopy, Practical Spectroscopy, ed., Vol. 28. 2001, Marcel Dekker, Inc.: New York, 1054.
Surface-enhanced Raman scattering (SERS) provides an enhancement in the Raman scattering signal by up to 106 to 1010 for molecules adsorbed on microstructures of metal surfaces. Background for this concept is described in Surface-Enhanced Spectroscopy, by Moskovits, M., Rev. Mod. Phys., 57(3): p. 783-828 (1985). The enhancement is due to a microstructured metal surface scattering process which increases the intrinsically weak normal Raman Scattering due to a combination of several electromagnetic and chemical effects between the molecule adsorbed on the metal surface and the metal surface itself.
The enhancement is primarily due to enhancement of the local electromagnetic field in the proximity of the molecule resulting from plasmon excitation at the metal surface. [Moskovits, M., Surface-Enhanced Spectroscopy, Rev. Mod. Phys., 1985. 57(3): p. 783-828; Kneipp, K., et al., Ultrasensitive Chemical Analysis by Raman Spectroscopy, Chem. Rev., 1999.99: p. 2957-2975]. Although chemisorption is not essential, when it does occur there may be further enhancement of the Raman signal, since the formation of new chemical bonds and the consequent perturbation of adsorbate electronic energy levels can lead to a surface-induced resonance effect. [Moskovits, M., Surface-Enhanced Spectroscopy, Rev. Mod. Phys., 1985. 57(3): p. 783-828; Kneipp, K., et al., Ultrasensitive Chemical Analysis by Raman Spectroscopy, Chem. Rev., 1999. 99: p. 2957-2975]. The combination of surface- and resonance-enhancement (SERS) can occur when adsorbates have intense electronic absorption bands in the same spectral region as the metal surface plasmon resonance, yielding an overall enhancement as large as 1010 to 1012. Kneipp, K., et al., Ultrasensitive Chemical Analysis by Raman Spectroscopy, Chem. Rev., 1999. 99: p. 2957-2975.
In addition to roughened metal surfaces, solid gold and silver nano-particles in a size range of approximately 40 nm to about 200 nm can also generate SERS. These particles support resonant surface plasmons that can be excited by light in the visible part of the optical spectrum, wherein the absorption maximum for such particles depends on a number of factors, such as material (e.g., gold, silver, copper), size, shape and the dielectric constant of the medium surrounding the particle.
[Yguerabide, J. and E. E. Yguerabide, Light-Scattering Submicroscopic Particles as Highly Fluorescent Analogs and Their Use as Tracer Labels in Clinical and Biological Applications, II. Experimental Characterization. Anal. Biochem., 1998. 262: p. 157-176; Yguerabide, J. and E. E. Yguerabide, Light-Scattering Submicroscopic Particles as Highly Fluorescent Analogs and Their Use as Tracer Labels in Clinical and Biological Applications, I. Theory. Anal. Biochem., 1998. 262: p. 137-156]. These properties make the particles useful as substrates for surface-enhanced Raman spectroscopy, which can increase the Raman-scattered signal by many orders of magnitude above that of conventional SERS approaches involving planar, roughened surfaces. Kneipp, K., et al., Ultrasensitive Chemical Analysis by Raman Spectroscopy, Chem. Rev., 1999. 99: p. 2957-2975.
Typically, molecules adsorb to the particles by charge-interaction (electrostatics) with the charged particles, or by covalent binding through sulfur-containing chemical groups (e.g., thiol-groups). Additional improvement in signal-to-noise occurs because the conducting nanoparticles tend to quench any natural fluorescence produced by the molecules. These increases in Raman signal have been shown to be large enough to allow the Raman spectra from single molecules to be obtained. [Kneipp, K., et al., Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS). Phys. Rev. Lett., 1997. 78(9): p. 1667-1670; Nie, S. and S. R. Emory, Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science, 1997. 275: p. 1102-1106]. This extraordinary increase in the Raman signal makes it possible to obtain Raman spectra rapidly and therefore makes it feasible to use Raman spectroscopy for real-time sensor applications.
Background information for systems and methods based on Raman and surface-enhanced Raman scattering (SERS) is described and claimed in U.S. Patent No. 2003/0059820 A1, entitled “SERS Diagnostic Platforms, Methods and Systems Microarrays, Biosensors and Biochips,” issued Mar. 27, 2003 to Vo-Dinh, including the following, “In a preferred embodiment of the invention, the sampling platform is a SERS platform, permitting the system to be a SERS sensor. The SERS sampling platform includes one or more structured metal surfaces. A plurality of receptor probes are disposed anywhere within the range of the enhanced local field emanating from the structured metal surfaces. The Raman enhancement occurs upon irradiation of the structured metal surfaces. Such receptor probe proximity permits SERS enhancement of the Raman signal from the receptor probe/target combination which is formed following a binding event . . . .”
A need exists for an improved Surface-Enhanced Raman Spectroscopy (SERS) method and apparatus/system to determine the presence and concentration of one or more target molecules, and/or changing physical and/or chemical conditions of an analyte. The present invention is directed to such a need.